1. Technical Field
The present disclosure relates to an output control circuit that performs feedback control through detection and generates a high-frequency output signal (radio frequency output signal).
2. Description of the Related Art
Recently, in high-speed transmission wireless communication, wireless communications devices transmit and receive high-frequency signals, i.e., signals in a high-frequency band, using a higher frequency band (e.g., a high-frequency band of 60 GHz or higher such as a millimeter-wave band) in order to perform faster transmission while ensuring a frequency band of modulation signals. Further, in the case of performing long-distance wireless communication, the levels of outputs signals need to be held constant both in the transmission characteristic of a transmitter side and the reception characteristic of a receiver side for stable maintenance of high-speed and high-quality communication.
For example, in the case of occurrence of external factors (e.g., temperature change and power supply variation) in the generation of a high-frequency signal, the power of the high-frequency signal needs to be constantly controlled. Therefore, an adjustment method has conventionally been employed that includes providing a detector circuit that detects a change in the power of a high-frequency signal and adjusting, according to a result of the detection by the detector circuit, the gain of a high-frequency amplifier circuit that amplifies the high-frequency signal.
Further, for example, in a case where a direct conversion scheme is used as an RF (radio frequency) configuration in a transmitting device to obtain a high-frequency signal in a high-frequency band such as a millimeter-wave band, an oscillator circuit generates a high-frequency signal in a millimeter-wave band, and the high-frequency signal thus generated is inputted to a mixer circuit. In a case where the oscillator circuit directly generates a high-frequency signal in a millimeter-wave band, the effects of frequency stability, in-band noise, and the like make it difficult for the conventional oscillator circuit to ensure the characteristic of the high-frequency signal. Therefore, a carrier signal in a high-frequency band is generated using an N-multiplier circuit that N-multiplies an input signal. Specifically, the oscillator circuit generates a signal in a low-frequency band (fundamental frequency band) with a good characteristic, and the N-multiplier circuit increases, to an N-multiplied frequency band, the signal in the fundamental frequency band that was generated by the oscillator, and generates a carrier wave in a high-frequency band.
The N-multiplier circuit, which N-multiplies an input signal, has two operation regions. One of the two operation regions is a linear region in which there is a linear relationship between the level of an input signal and the level of an output signal, and the other operation region is a saturation region in which the level of an output signal is saturated with respect to the level of an input signal.
For example, in a case where the oscillator circuit and a 2-multiplier circuit are used to generate a carrier signal in an 80 GHz frequency band, the frequency of a signal that is inputted to the 2-multiplier circuit is 40 GHz. Also, in a case where the oscillator circuit and a 4-multiplier circuit are used to generate a carrier signal in an 80 GHz frequency band, the frequency of a signal that is inputted to the 4-multiplier circuit is 20 GHz. In a case where a signal that is inputted to the N-multiplier circuit is a high-frequency signal, the gain characteristic of a transistor that is used in an input amplifier circuit of the N-multiplier circuit is insufficient. This may result in a greater change in gain characteristic due to external factors (e.g., temperature change and power supply variation) so that the operation region of the N-multiplier circuit may be the linear region. In a case where the N-multiplier circuit operates in the linear region, a variation in the level of a high-frequency signal that is N-multiplied by and outputted from the N-multiplier circuit is N times greater than a variation in the level of a signal that is inputted to the N-multiplier circuit in the N-multiplication settings, and the variation (change) in the output level increases.
This makes it necessary to provide a detector circuit that accurately detects a change in the output level of a high-frequency signal that is outputted from the N-multiplier circuit and to perform feedback control so that the level of the high-frequency signal may become constant. However, in a high-frequency band such as a millimeter-wave band, a source of generation of a reference signal for calibrating the detector circuit operates in the millimeter-wave band, too. This results in a greater variation in the level of the reference signal due to external factors (e.g., temperature change and power supply variation). Furthermore, greater variations in the gain characteristic and sensitivity characteristic of the detector circuit per se, which operates in the millimeter-wave band, make it difficult to detect a change in the output level of the high-frequency signal.
Determination of the operating state (linear operation/saturated operation) of a high-frequency circuit such as the N-multiplier circuit can be made on the basis of the ratio of an output level change (ΔPout) to a constant input level change (ΔPin). For example, in a case where it is desirable that the N-multiplier circuit be controlled as a high-frequency amplifier in a region (saturated operation region) that is equal to or higher than the input level of a 1-dB gain suppression point, determination of the 1-dB gain suppression point (P1 dB) can be made on the basis of ΔPout/ΔPin≦N [dB]. The term “1-dB gain suppression point” here means a point at which the output level drops by 1-dB with respect to the theoretical output level in a case where the amplifier has a linear gain characteristic.
However, in the case of occurrence of a variation in the output level of a high-frequency signal, there is also a greater variation in the output level change (ΔPout), which is a result of detection of an output signal. In a case where the output level change (ΔPout) is 1 [dB] or greater, it is difficult to accurately make determination of the operating state (linear operation/saturated operation) as described above. In a case where accurate determination is not made and the N-multiplier circuit linearly operates due to external factors such as temperature change, the high-frequency signal N-multiplied by the N-multiplier circuit is linearly amplified with respect to the signal that is inputted to the N-multiplier circuit. In this case, the control of gain by the high-frequency amplifier circuit expands the required range of gain control, thus causing an increase in circuit size and an increase in consumption current.
This makes it necessary to reduce variations in the output levels of high-frequency signals due to external factors (e.g., temperature change and power supply variation). For example, Japanese Patent No. 5206828 discloses a control circuit 100 that controls the output level of a high-frequency signal. FIG. 1 schematically shows a configuration of the control circuit 100.
However, with the conventional control circuit 100 described in Japanese Patent No. 5206828, which is shown in FIG. 1, there may be great variations in the gain characteristics of high-frequency amplifiers and the sensitivity characteristics of detector circuits from circuit to circuit due to temperature change. In this case, use of the same temperature correction data that is held by temperature correction controllers leads to greater variations in the levels of high-frequency signals from circuit to circuit. Further, even in the case of acquisition of temperature correction data for each control circuit at the time of initial calibration, changes over time leads to changes in the gain characteristics of the high-frequency amplifiers, thus leading to greater variations in the levels of the high-frequency signals. Therefore, there has been a demand for a countermeasure.
In a circuit that includes an N-multiplier circuit in order to obtain a high-frequency signal and operates in a frequency band, such as a millimeter-wave band, in which there is a great variation in characteristic due to external factors (e.g., temperature change and power supply variation), a configuration in which an N-multiplied high-frequency signal is detected by a detector circuit cannot determine the operation region (linear region, saturation region) of the N-multiplier circuit or control the N-multiplier circuit in the saturated operation region. In a case where the N-multiplier circuit operates in the linear region, a variation in output signal level is N times greater than a variation in input signal level. The expansion in the adjustable range of gain of the high-frequency amplifier causes an increase in circuit size and an increase in consumption current.